A bolometer is a temperature sensor that can be used as an infrared radiation sensing device, well known in the art. The bolometer senses the incident infrared radiation by changing its temperature. The change in temperature causes a change in the electrical resistance of the bolometer. The resistance of the bolometer either increases or decreases, depending on the material the bolometer is made off. According to a simple model of a bolometer, wherein only incident infrared radiation is considered, the infrared radiation energy can be determined by measuring the change of the bolometer resistance. It is noted that the temperate of a bolometer may further change (i.e., other that radiation) by exchanging heat energy with the surroundings (e.g., air or electrical contacts) of the bolometer, via conduction. The temperature of a bolometer may further change due to current, flowing through a bolometer, during the measurement process. The change in temperature, due to current flow, is known as self-heating. In calculating the power dissipation in a bolometer, the noted phenomena should be accounted for.
A microbolometer is small bolometer usually in the order of a few tens of microns in size. Microbolometers are commonly used in infrared imaging devices. An introduction to bolometers is provided by “Low-Cost Uncooled Infrared Detectors in CMOS Process” by Eminoglu et. al, available at www.sciencedirect.com.
Reference is now made to FIG. 1A, which is a schematic illustration of an exemplary thermal imaging microbolometer apparatus, generally referenced 2, which is known in the art. Apparatus 2 measures the resistance of a bolometer using a reference voltage, according to a simple model. This simple model operates under the assumption that the measurement activity, negligibly changes the bolometer temperature, due to the energy dissipated during the measurement process.
Apparatus 2 includes a reference voltage source 4 and a bolometer 6. Bolometer 6 is coupled in parallel with reference voltage source 4. Voltage source 4, supplies a constant voltage across bolometer 6. Infrared radiation 8, incident on bolometer 6, causes the resistance of bolometer 6 to change. Consequently, when the reference voltage is applied to the bolometer, the current through bolometer 6, Iout, changes. The resistance of bolometer 6, is determined by measuring Iout with a current measuring device, known in the art (not shown) and applying the law of Ohm. The law of Ohm, known in the art, is stated in Equation (1)
                              R          b                =                  V          I                                    (        1        )            wherein, Rb is the resistance of the bolometer, V is the voltage across the bolometer and I is the current through the bolometer. In apparatus 2, the voltage across bolometer 6 is Vref and the current through bolometer 6 is Iout.
Reference is now made to FIG. 1B, which is a schematic illustration of an exemplary thermal imaging microbolometer apparatus, generally referenced 10, which is known in the art. Apparatus 10, measures the resistance of a barometer, using a reference current, according to the model of FIG. 1A.
Apparatus 10 includes a reference current source 12 and a bolometer 14. Bolometer 14 is coupled in parallel with current source 12. Current source 12 provides a constant current, Iref, through bolometer 14. Infrared radiation 16, incident on bolometer 14, causes the resistance of bolometer 14 to change. Consequently, when the reference current is applied to the bolometer, the voltage, across bolometer 14, Vout, changes. The resistance of bolometer 14, is determined by measuring Vout with a voltage measuring device, known in the art (not shown) and applying the law of Ohm, stated in equation (1). In apparatus 10, V is the voltage across bolometer 14, Vout and I is the reference current Iref.
Reference is now made to FIG. 1C, which is a schematic illustration of an exemplary thermal imaging microbolometer apparatus, generally referenced 20, which is known in the art. Apparatus 20, measures the resistance of a bolometer, using a reference current created by a voltage source and a reference resistor, according to the model of FIG. 1A. Apparatus 20 includes a voltage source 22, a bolometer 24 and a reference resistor 26. Reference resistor 26, is coupled in series with bolometer 24. Voltage source 22 is coupled with the series combination of reference resistor 26 and bolometer 24. Voltage source 22 supplies a constant voltage across the series combination of reference resistor 26 and bolometer 24.
Infrared radiation 28, incident on bolometer 24, changes the resistance of bolometer 24. Consequently, when the reference current is applied to the bolometer, the voltage across bolometer 24, Vout, changes. The resistance of bolometer 24, is determined by measuring Vout. Vout is measured with a voltage measuring device, known in the art (not shown) and applying equation (2)
                              R          b                =                                            V              out                        ⁢            R                                V            -                          V              out                                                          (        2        )            wherein V is the voltage across the series combination of the bolometer and the reference resistor, Vout is the voltage across the bolometer and R is the resistance of the reference resistor. In apparatus 20, V is voltage source 22, R is the resistance of reference resistor 26 and Vout is the voltage measured across bolometer 24.
Measuring signals (e.g., current, voltage) in an electrical system is a process accompanied by electrical disturbances known as noise. Noise may cause errors in the measurement. It is therefore desirable to increase the signal and reduce the noise (i.e., increase the signal to noise ration). In order to increase the signal to noise ration (i.e., SNR) of the bolometer signal measurement, a more common technique involves measuring the accumulated current through or voltage across the bolometer over time (i.e., integrating the current through the bolometer over a predetermined time period). The integration operation in effect “averages” the measured signal (i.e., the desired signal and the noise), over the predetermined time period.
Reference is now made to FIG. 1D, which is a schematic illustration of a thermal imaging microbolometer apparatus, generally referenced 30, which is known in the art. Apparatus 30, integrates the current through the bolometer over a predetermined time period. The current through the bolometer is related to the resistance of the bolometer by equation (1). The resistance of the bolometer may change due to incident radiation. Apparatus 30 may be a part of an integrated circuit including several microbolometers. Apparatus 30 includes three voltage sources 32, 34 and 36, a bolometer 38, a reference resistor 40, a pMOS transistor 44, an nMOS transistor 46 and an integrator 48. Integrator 48, known in the art, further includes an amplifier 50, a feedback capacitor 52 and a switch 56.
The negative terminal of voltage source 32 is coupled with ground. The positive terminal of voltage source 32 is coupled with one of the terminals of bolometer 38. The other terminal of bolometer 38 is coupled with the source terminal of pMOS transistor 44. The drain terminal of pMOS transistor 44 is coupled with the drain terminal of nMOS transistor 46. The source terminal of nMOS transistor 46 is coupled with one of the terminal of reference resistor 40. The other terminal of reference resistor 40 is coupled with the negative terminal of voltage source 34. The positive terminal of voltage source 34 is coupled with ground. The negative terminal of voltage source 36 is coupled with ground and the positive terminal of voltage source 36 is coupled with the positive input terminal of amplifier 50. The negative input terminal of amplifier 50 is coupled with the drain terminals of pMOS transistor 44 and nMOS transistor 46. One of the terminals of feedback capacitor 52 and switch 56 are coupled with the negative input terminal of amplifier 50. The other terminals of feedback capacitor 52 and switch 56 are coupled with the output terminal of amplifier 50.
Voltage source 32 provides a reference voltage and thus a reference current to the circuit. Voltage source 34 provides the bias value of the voltage across the negative input terminal of amplifier 50 and ground. Voltage source 36 sets the bias value of the voltage across the positive input terminal of amplifier 50 and ground. pMOS transistor 44 provides a mean to control the reference current through bolometer 38 and nMOS transistor 46 in conjunction with resistor 40 provides a mean to reduce the value of the current through the bolometer and to compensate for temperature fluctuations in the immediate surrounding of the microbolometer (e.g., temperature fluctuations of the substrate in an integrated circuit). Resistor 40 is a thermally shorted microbolometer typically used in microbolometer readout circuits. Switch 56 controls the integration period.
Incident radiation 42, changes the resistance of bolometer 38. Thus, the current through bolometer 38 changes during the integration operation. Amplifier 50 may be an operational amplifier, known in the art wherein, the voltage difference across its negative and positive input terminals is approximately zero causing the charge flow into the negative terminal of amplifier 50 to accumulate on capacitor 56. Thus, Integrator 48, integrates the current through the negative terminal of amplifier 50 over a predetermined period, controlled by switch 56. Consequently, the output of integrator 48 relates to the accumulated incident energy on bolometer 38.
Microbolometers for infrared imaging are typically fabricated on integrated circuits in a two dimensional N×M array of microbolometers, each functioning as a single picture element known as a pixel. The array senses the change in temperature of a focal plane of incident radiation. The change in the resistance, of each microbolometer in the array, is translated to an electrical signal as described above. The electrical signal can, be transferred, for example, to an imaging device for display or to a memory device for storage.
A Read Out Integration Circuit (ROIC) is an electrical circuit for integrating the electrical signal resulting from the incident radiation in a microbolometer pixels. The term “Retrieval” relates to the operation of integrating the electrical signal relating to incident radiation on the microbolometer pixel and releasing. The term “Releasing” relates to transferring the resulting integrated electrical signal out of an ROIC module. The term “Resetting” is the operation of clearing the ROIC from previous values (i.e., the integrated electrical signals), to ensure the integrity of the values resulting from the next integration operation. An ROIC is a device performing retrieval.
An array of microbolometer pixels in combination with an ROIC module or modules is known as a microbolometer Focal Plane Array (FPA). The time period of retrieving an entire two dimensional array of microbolometer pixels, is known as a frame acquisition retrieval period, and is denoted Q. During the frame acquisition period, the microbolometer pixel is exposed to incident radiation for an ample portion of Q, known as exposure period. During exposure period, the resistance of the microbolometer changes to a value related to the temperature of the focal plane of the incident radiation. For the remaining portion of the frame acquisition period, the microbolometer pixel is retrieved. During the microbolometer pixel retrieval period, reference current is applied through the microbolometer. The reference current is applied through the microbolometer in order to acquire a measurement relating to the resistance of the microbolometer, and consequently to the incident radiation on the microbolometer. The measurement is integrated over a predetermined time period to increase the SNR.
A pixel in an FPA may be retrieved by selecting a row of pixels, enabling retrieval of all the pixels in the selected row. A specific pixel, to be retrieved, is selected from the enabled row. Thus, the pixel is coupled with the ROIC module. The ROIC module retrieves the selected pixel. This process is repeated until all the pixels are retrieved. Accordingly, the pixel retrieval time period is the frame acquisition period Q, divide by the number of pixels M×N (i.e., Q/M×N). Releasing and resetting the ROIC module takes a few microseconds. Thus, the actual integration period slightly less than Q/M×N. The microbolometer exposure period to incident radiation is thus slightly less than Q.
Johnson noise is the noise generated by thermal agitation of electrons in a conductor. Johnson noise is a dominant source of noise in microbolometers that might degrade the performance of FPA systems. Johnson noise is usually more dominant than other noise sources such as KTC noise and thermal fluctuation noise. The Johnson noise is proportional to the square root of the bandwidth of the system. The bandwidth of the system is controlled by the integration period of the ROIC. An expression for the Johnson noise in degrees Kelvin is given in Equation (3)
                              Δ          ⁢                                          ⁢                      T            johnson_noise                          =                              1                                          I                d                            ⁢              α                                ⁢                                                    4                ⁢                                  kT                  d                                                                              R                  b                                ⁢                                  T                  i                                                                                        (        3            wherein Id is the current through the microbolometer, Td is the temperature in degrees Kelvin of the microbolometer, Rb is the resistance of the microbolometer, Ti is the readout integration period during which a measurement relating to the resistance of the microbolometer is acquired, a is the temperature coefficient of resistance of the bolometer and k is Boltzmann's constant. Reducing the bandwidth or alternately increasing the integration period reduces the Johnson noise and consequently improves the performance of the FPA system.
U.S. Pat. No. 5,698,852 issued to Tanaka et al entitled “Titanium Bolometer-Type Infrared Detecting Apparatus” is directed to a method wherein two ROICs are used in order to increase the integration period. The integration period is increased to slightly less than 2Q/M×N. A pixel in the array is retrieved by selecting a row of pixels, enabling the retrieval of all the pixels in the row. A specific pixel, to be retrieved, is selected from the enabled row. Initially, the first ROIC module is allocated to a selected pixel. The selected pixel is coupled with the first ROIC module. After a delay of Q/M×N, the second ROIC module is allocated to another selected pixel. The other selected pixel is coupled with the second ROIC module. After a delay of slightly less than Q/M×N the first ROIC module is released and reset. The next pixel in the row is selected and coupled with the first ROIC module. After a delay slightly less than Q/M×N the second ROIC module is released and reset. The next pixel in the row is selected and coupled with the first ROIC module. This process is repeated until all the rows of the pixel array are retrieved. Consequently, integration time period is increased to slightly less than 2Q/M×N for each pixel.
U.S. Pat. No. 5,965,892 issued to Tanaka entitled “Thermal-Type Infrared Imaging Device”, is directed to a method of operation, similar to that described in U.S. Pat. No. 5,698,852. In addition, Tanaka describes a row select circuit selects only odd rows during the first half of the integration period. Even rows are retrieved during the second half the integration period.
U.S. Pat. No. 6,028,309 issued to Parrish et al entitled “Method and Circuitry for Correcting Temperature-induced Errors in Microbolometer Focal Plane Array” is directed to a method of simultaneously integrating all the pixels in a row, from an array of microbolometer pixels.
Reference is now made to FIG. 2, which is a schematic illustration of an FPA system, generally referenced 70, which is known in the art. System 70 integrates a row of pixels, from an array of pixels, with a row of M ROIC modules. System 70 includes an N×M array 72 of microbolometer pixels, a row select circuit 74 and a row of M ROIC modules 76 also referred to as ROIC row. Row select circuit 74, is operative to select all the pixels of a specific row, to be retrieved. Thus, each pixel element in the selected row is coupled with a respective ROIC module in ROIC row 76. ROIC row 76 integrates the entire selected row for a period of slightly less than Q/N. After integration is completed, the signals are released. The ROIC modules are reset. Row select circuit 74 selects the next row of microbolometer pixels to be retrieved. This operation is repeated, until all the rows of array 72 are retrieved.
Reference is made to FIG. 3, which is an illustration of a timing scheme, generally referenced 80, of FPA system 70 in FIG. 2, which is known in the art. FIG. 3 shows, HORIZONTAL DRIVE 82, ROIC ROW signal 84 and a period 86. HORIZONTAL DRIVE represents a signal of time period Q/N that drives the row select circuit. ROIC ROW signal 84 represents the retrieval period of ROIC row 76 in FIG. 2. Period 86, at the end of the retrieval period is the time duration of releasing and resetting the row of ROIC modules.